Figure 2. The mean score of imagery valence rating. Error bars represent the standard error. The asterisks indicate significant differences among conditions (**p <.01, *p <.05).

3.2. ERP results

Repeated-measures ANOVA analysis of LPP amplitudes revealed a significant interaction between group and emotion (F (2, 164)=13.40,p <0.001,ηp2 =0.14). Simple effects analyses showed that depressed participants exhibited larger LPP for sad imagery compared to happy imagery (p =0.008); no significant differences were found for sad (p =0.05) and happy imagery (p =1.00) compared to neutral imagery. Conversely, healthy controls exhibited greater LPP for happy imagery than for sad imagery (p <0.001); no significant differences were found for happy(p =0.08) and sad imagery(p= 0.17) compared to neutral imagery. Additionally, depressed individuals exhibited significantly larger LPP for sad imagery compared to healthy controls( p =0.03). The main effect of the time window was significant (F (2, 164)=14.05, p <0.001,ηp2 =0.15), and wherein the LPP amplitudes were more positive in the early (p =0.003) and middle (p <0.001) time windows compared to the late time window. The main effects of group and emotion, as well as the interaction effects of group and time window, emotion and time window, were not significant (ps > 0.23).
The interaction effect of group × emotion × time window was significant (F (4, 328)=14.05, p =0.03,ηp2 =0.03), followed up by separate analyses of the three time windows.
Early LPP amplitudes. The results showed a significant interaction effect between group and emotion (F (2, 164) =11.07,p <0.001,ηp2 =0.12). Simple effect analysis showed that depressed participants exhibited significantly larger early LPP for sad imagery compared to both happy ( p =0.03) and neutral imagery ( p =0.02). Conversely, healthy controls exhibited greater early LPP for happy imagery than for sad imagery (p =0.001), and greater but non-significant than neutral imagery (p =0.18). No other effects reached significance (ps >0.39).
Middle LPP amplitudes. The results revealed a significant interaction effect between group and emotion (F (2, 164)=12.40,p <0.001,ηp2 =0.13). Simple effect analysis showed that depressed participants exhibited significantly larger middle LPP for sad imagery compared to both happy (p =0.003) and neutral imagery (p =0.02). Conversely, healthy controls exhibited greater middle LPP for happy imagery compared to sad imagery (p =0.003), and greater but non-significant than neutral imagery( p =0.07). Additionally, depressed individuals exhibited larger middle LPP for sad imagery relative to healthy controls (p =0.02). No other effects reached significance (ps >0.29).
Late LPP amplitudes. The results revealed a significant interaction effect between group and emotion (F (2, 164)=10.73,p <0.001,ηp2 =0.12). Simple effect analysis showed that depressed participants exhibited significantly larger late LPP for sad imagery compared to happy imagery (p =0.04), but no significant difference was observed between late LPP for happy imagery and neutral imagery (p =0.56). For the control group, happy imagery elicited greater late LPP than sad imagery (p =0.001), and greater but non-significant than neutral imagery (p =0.17). Furthermore, the depression group exhibited significantly larger late LPP for sad imagery compared to the control group (p =0.02). No other effects reached significance (ps >0.45). See Figure 3 and Figure 4.